Powder coating materials are a cost-effective and environmentally friendly alternative to liquid paints. Crosslinking of the binders to form a protective paint film is generally effected by thermal activation, the coated articles being subjected in an oven to a temperature of customarily from about 160 to about 200° C. As the substrate is heated, the powder melts, within a temperature range of customarily from about 80 to about 120° C., and runs out to form a uniform film, which on further heating (from about 110 to about 140° C.) finally cures as a result of the onset of the thermally activated crosslinking reaction. Depending on the reactivity of the system used, complete crosslinking takes between 10 and 30 minutes.
Since there is an overlap between the temperature ranges of the melting process and the crosslinking process, and since as well the heating of the powder coating layer on the substrate is never entirely homogeneous, melting and crosslinking occasionally take place simultaneously; the beginning of crosslinking is also coupled with a drastic increase in the viscosity, which impedes flow and hence the formation of an even surface. In practice, therefore, uneveness in the coating is frequently observed: terms such as “orange peel” and “craters” are used where the particle structure of the applied powder coating material is still more or less visible. Where the substrate is insufficiently wetted by the melted coating material, small punctures can be seen in the coating, which look like pricks from a needle and are therefore generally referred to as “pin holes”.
It is therefore an object of the invention to modify powder coating materials such that during the melting process the viscosity is as low as possible and an appropriate surface tension is obtained without signs of running (“curtains”) being visible on inclined or vertical surfaces.
This object can be achieved in accordance with the invention by adding certain additives to powder coating materials, the additives used being certain fluoro- and hydroxyl group-containing polymers. These additives enhance the levelling of the coating materials and prevent or render less visible the surface defects described above.
Fluoro- and hydroxyl group-containing copolymers are known from EP-A 0 849 283.
They are in the form of nonaqueous dispersions, and are combined with hydroxyl group-containing resins and crosslinkers together to form coating compositions. Their use as additives to powder coating materials is not described.
Copolymers of hydroxyalkyl acrylates and fluoroalkyl acrylates are also known from WO-A 97/01 114, EP-A 0 620 455, EP-A 0 563 047, and EP-A 0 167 634, and are used for contact lenses. The use of these copolymers as additives to powder coating materials is not described.
Moreover, the use of such copolymers as additives for anionically stabilized aqueous coating compositions (EP-A 0 362 643) and (copolymerized with certain silanes) as photopolymerizable adhesives (EP-A 0 583 471) is known. Here again, their use as additives to powder coating materials is not described.